Lithium silicate materials

ABSTRACT

Lithium silicate materials are described which can be easily processed by machining to dental products without undue wear of the tools and which subsequently can be converted into lithium silicate products showing high strength.

This application is a division of U.S. patent application Ser. No.10/913,095, filed Aug. 6, 2004, which claims priority to German PatentApplication Serial No. 103 36 913.9, filed Aug. 7, 5, 2003, which areherein incorporated by reference in their entirety.

The invention relates to lithium silicate materials which can be easilyshaped by machining and subsequently converted into shaped products withhigh strength.

There is an increasing demand for materials which can be processed intodental restorative products, such as crowns, inlays and bridges, bymeans of computer controlled milling machines. Such CAD/CAM methods arevery attractive as they allow to provide the patient quickly with thedesired restoration. A so-called chair-side treatment is thus possiblefor the dentist.

However, materials suitable for processing via computer aideddesign/computer aided machining (CAD/CAM) methods have to meet a veryspecific profile of properties.

First of all, they need to have in the finally prepared restorationappealing optical properties, such as translucence and shade, whichimitate the appearance of the natural teeth. They further need to showhigh strength and chemical durability so that they can take over thefunction of the natural tooth material and maintain these propertiesover a sufficient period of time while being permanently in contact withfluids in the oral cavity which can even be aggressive, such as acidicin nature.

Secondly and very importantly, it should be possible to machine them inan easy manner into the desired shape without undue wear of the toolsand within short times. This property requires a relatively low strengthof the material and is therefore in contrast to the desired propertiesmentioned above for the final restoration.

The difficulty of combining the properties of low strength in the stageof the material to be processed and a high strength of the finalrestoration is reflected by the known materials for a CAD/CAM processingwhich are in particular with respect to an easy machinabilityunsatisfactory.

DE-A-197 50 794 discloses lithium disilicate glass ceramics which areprimarily intended to be shaped to the desired geometry by ahot-pressing process wherein the molten material is pressed in theviscous state. It is also possible for these materials to be shaped bycomputer aided milling processes. However, it has been shown that themachining of these materials results in a very high wear of the toolsand very long processing times. These disadvantages are caused by thehigh strength and toughness primarily imparted to the materials by thelithium disilicate crystalline phase. Moreover, it has been shown thatthe machined restorations show only a poor edge strength. The term“ledge strength” refers to the strength of parts of the restorationhaving only a small thickness in the range of few 1/10 mm.

Further approaches of achieving easy machinability together with a highstrength of the final restoration have also been made. EP-B-774 993 andEP-B-817 597 describe ceramic materials on the basis of Al₂ 0 ₃ or ZrO₂which are machined in an unsintered state which is also referred to as“green state”. Subsequently, the green bodys are sintered to increasethe strength. However, these ceramic materials suffer from a drasticalshrinkage of up to 50% by volume (or up to 30% as linear shrinkage)during the final sintering step. This leads to difficulties in preparingthe restorations with exactly the dimensions as desired. The substantialshrinkage represents a particular problem if complicated restorationsare manufactured, such as a multi-span bridge.

From S. D. Stookey: “Chemical Machining of Photosensitive Glass”, Ind.Eng. Chem., 45, 115-118 (1993) and S. D. Stookey: “PhotosensitivelyOpacifiable Glass” U.S. Pat. No. 2,684,911 (1954) it is also known thatin lithium silicate glass ceramics a metastable phase can be formed atfirst. For example in photosensitive glass ceramics (Fotoform®,FotoCeram®) Ag-particles are formed using UV-light. These Ag-particlesserve as crystallization agent in a lithium metasilicate phase. Theareas which were exposed to light are in a subsequent step washed out bydiluted HF. This procedure is possible since the solubility of thelithium metasilicate phase in HF is much higher than the solubility ofthe parent glass. The glass portion remaining after said solubilizingprocess (Fotoform®) can be transferred into a lithium disilicate glassceramic (FotoCeram®) by an additional heat treatment.

Also investigations of Borom, e.g. M.-P. Borom, A. M. Turkalo, R. H.Doremus: “Strength and Microstructure in Lithium DisilicateGlass-Ceramics”, J. Am. Ceream. Soc., 58, No. 9-10, 385-391 (1975) andM.-P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahren zum Herstellen vonClaskeramiken” DE-A-24 51 121 (1974), show that a lithium disilicateglass ceramic can in the first instance crystallize in varying amountsas metastable lithium metasilicate phase. However, there also existcompositions which crystallize in the form of the disilicate phase fromthe beginning and the metasilicate phase is not present at all. Asystematic investigation of this effect has not become known. From theinvestigations of Borom it is also known that the glass ceramic whichcontains lithium metasilicate as the main phase has a reduced strengthcompared to the one of a glass ceramic which only contains a lithiumdisilicate phase.

Thus, the prior art materials show a couple of shortcomings. It is,therefore, an object of the present invention to eliminate thesedisadvantages and in particular to provide a material which, above all,can be easily shaped by computer-aided milling and trimming processesand can subsequently be converted into high-strength dental productswhich also display a high chemical durability and excellent opticalproperties and exhibit a drastically reduced shrinkage during said finalconversion.

This object is achieved by the lithium silicate glass ceramic material.

The invention also relates to a lithium disilicate material, a dentalproduct, processes for the preparation of a lithium silicate blank and adental restoration according to, a lithium silicate glass, a blank, andto methods for manufacturing a lithium silicate restoration or a dentalrestoration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a principle temperature profile of a process according tothe invention starting from the melt via lithium metasilicate to lithiumdisilicate.

FIG. 2 shows a DSC-plot of a lithium silicate material according toexample 13.

FIG. 3 shows a high temperature XRD of a lithium silicate materialaccording to example 13, in form of a bulk glass sample.

FIG. 4 shows an XRD for phase analysis of a lithium silicate materialaccording to example 13 after nucleation and first crystallization.

FIG. 5 shows an SEM-micrograph, back scattered electrons, of a lithiumsilicate material according to example 13 after nucleation and firstcrystallization.

FIG. 6 shows an XRD for phase analysis of a lithium silicate materialaccording to example 13 which was subjected to nucleation, firstcrystallization and second crystallization conditions, and

FIG. 7 shows an SEM-micrograph, back scattered electrons, of a lithiumsilicate material according to example 13 which was subjected tonucleation, first crystallization and second crystallization conditionsand has an etched surface.

It has surprisingly been shown that by using a starting glass of a veryspecific composition and a specific process it is possible to providethe glass ceramic according to the invention which has metastablelithium metasilicate (Li₂SiO₃) as main crystalline phase rather thanlithium disilicate (Li₂Si₂ 0 ₅). This lithium metasilicate glass ceramichas a low strength and toughness and hence can be easily machined intothe shape of even complicated dental restorations, but can after suchmachining be converted by a heat treatment into a lithium disilicateglass ceramic product with outstanding mechanical properties, excellentoptical properties and very good chemical stability thereby undergoingonly a very limited shrinkage.

The lithium silicate glass ceramic material according to the inventioncomprises the following components

Component Wt. % SiO₂ 64.0-73.0 Li₂O 13.0-17.0 K₂0 2.0-5.0 Al₂O₃ 0.5-5.0P₂O₅ 2.0-5.0and comprises lithium metasilicate as main crystalline phase.

Another preferred embodiment of the present invention is formed by asilicate glass ceramic material as described above which is formed in aprocess which includes a step wherein lithium metasilicate as maincrystalline phase is produced.

It is preferred that the lithium silicate material of the presentinvention further comprises the following additional componentsindependently from each other

Component Wt. % ZnO 0.5-6.0, preferably 2.0-6.0 Na₂O 0.0-2.0 Me^(II)O0.0-7.0, preferably 0.0-5.0 ZrO₂ 0.0-2.0 colouring and fluorescent 0.5-7.5, metal oxideswith Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO.

A lithium silicate material which comprises the following components,independently of one another, in the following amounts is particularlypreferred:

Component wt. -% SiO₂ 65.0-70.0 Li₂O 14.0-16.0 K₂O 2.0-5.0 Al₂O₃ 1.0-5.0P₂O₅ 2.0-5.0 ZnO 2.0-6.0 Na₂O 0.1-2.0 Me^(II)O 0.1-7.0, preferably0.1-5.0 ZrO₂ 0.1-2.0 coloring and fluorescent 0.5-3.5 metal oxideswith Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO, andwith the metal of the one or more coloring and fluorescent metal oxidesbeing preferably selected from the group consisting of Ta, Tb, Y, La,Er, Pr, Ce, Ti, V, Fe and Mn.

The phrase “. . . independently from each other . . . ” means that atleast one of the preferred amounts is chosen and that it is thereforenot necessary that all components are present in the preferred amounts.

As colouring components or fluorescent components for example oxides off-elements may be used, i.e. the list of metals given above is not to beseen as terminal. The colouring or fluorescent components ensure thatthe colour of the final dental product matches that of the natural toothmaterial of the patient in question.

In the above composition P₂ 0 ₅ acts as a nucleation agent for thelithium metasilicate crystals and a concentration of at least 2 wt % isrequired for the necessary nucleation. Instead of P₂ 0 ₅ othernucleation agents are also possible, e.g. compounds of the elements Pt,Ag, Cu and W.

In addition to the components mentioned above the glass ceramic mayfurther comprise additional components to enhance the glass technicalprocessability. Such additional components may therefore be inparticular compounds such as B₂ 0 ₃ and F which in general amount to 0to 5.0% by weight.

A lithium silicate material as described above is particularly preferredwhich comprises 67.0 to 70.0 wt % of SiO₂.

It has surprisingly been shown that a specific volume portion of lithiummetal silicate should be present to achieve excellent processingproperties. Thus, it is further preferred that the lithium metasilicatecrystalline phase forms 20 to 50 vol % and in particular 30 to 40 vol %of the lithium silicate material. Such a part of the volume leads to thecrystals being present rather remote from each other and hence avoids atoo high strength of the lithium silicate material.

The lithium metasilicate crystals are preferably of lamellar or plateletform. This leads to a very good machinability of the lithium silicatematerial without use of high energy and without uncontrolled breaking.The latter aspect of uncontrolled breaking is for example known fromglasses which are generally unsuitable for machining. It is assumed thatthe preferred morphology of the lithium metasilicate crystals is alsoresponsible for the surprisingly high edge strength of products, e.g.complicated dental restorations, can be made from the lithium silicatematerial according to the invention.

The lithium silicate material according to the invention preferably isin the form of a blank. The blank usually takes the form of a smallcylinder or a rectangular block. The exact form depends on the specificapparatus used for the desired computer-aided machining of the blank.

After the machining, the lithium silicate material according to theinvention has preferably the shape of a dental restoration, such as aninlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, acrown, a partial crown, a framework or a coping.

A lithium disilicate material which is formed in a process whichincludes a step wherein a phase comprising primarily crystalline lithiummetasilicate is produced, the lithium metasilicate being subsequentlyconverted to lithium disilicate forms a preferred embodiment of theinvention.

A dental product made from lithium disilicate, said lithium disilicatebeing formed in a process which includes a step wherein a phasecomprising primarily crystalline lithium metasilicate is produced, thelithium metasilicate being subsequently converted to lithium disilicateforms another preferred embodiment of the present invention.

A blank of lithium silicate glass ceramic material according to theinvention is preferably prepared by a process which comprises

-   -   (a) producing a melt of a starting glass containing the initial        components SiO₂, Li₂O, K₂ 0, Al₂ 0 ₃ and P₂ 0 ₅ as the main        components,    -   (b) pouring the melt of the starting glass into a mould to form        a starting glass blank and cooling the glass blank to room        temperature,    -   (c) subjecting the starting glass blank to a first heat        treatment at a first temperature to give a glass product which        contains nuclei suitable for forming lithium metasilicate        crystals,    -   (d) subjecting the glass product of step (c) to a second heat        treatment at a second temperature which is higher than the first        temperature to obtain the lithium silicate blank with lithium        metasilicate crystals as the main crystalline phase.

A process as described above, wherein the starting glass of step (a)further comprises ZnO, Na₂O, Me^(II)O, ZrO₂, and coloring andfluorescent metal oxides, with Me^(II)O being one or more membersselected from the group consisting of CaO, BaO, SrO and MgO ispreferred.

A process as described above, wherein the starting glass of step (a)comprises the following initial components, independently of oneanother, in the following amounts

Component wt. -% SiO₂ 65.0-70.0 Li₂O 14.0-16.0 K₂O 2.0-5.0 Al₂O₃ 1.0-5.0P₂O₅ 2.0-5.0 ZnO 2.0-6.0 Na₂O 0.1-2.0 Me^(II)O 0.1-7.0, preferably0.1-5.0 ZrO₂ 0.1-2.0 coloring and fluorescent 0.5-3.5 metal oxideswith Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO andwith the metal(s) of the one or more coloring and fluorescent metaloxides being preferably selected from the group consisting of Ta, Tb, Y,La, Er, Pr, Ce, Ti, V, Fe and Mn is even more preferred.

In step (a), a melt of a starting glass is produced which contains thecomponents of the glass ceramic. For this purpose a correspondingmixture of suitable starting materials, such as carbonates, oxides, andphosphates, is prepared and heated to temperatures of, in particular1300 to 1600° C., for 2 to 10 hours. In order to obtain a particularlyhigh degree of homogeneity, the glass melt obtained may be poured intowater to form glass granules and the glass granules obtained are meltedagain.

In step (b), the melt of the starting glass is poured into acorresponding mould, e.g. a steel mould, and cooled to room temperatureto give a glass product.

The cooling is preferably conducted in a controlled manner so as toallow a relaxation of the glass and to prevent stresses in the structureassociated with rapid temperature changes. As a rule, the melt istherefore poured into preheated moulds, e.g. of a temperature of about400° C. Subsequently, the product can slowly be cooled in a furnace toroom temperature.

In step (c) the starting glass product is subjected to a first heattreatment at a first temperature to cause formation of nuclei forlithium metasilicate crystals. Preferably, this first heat treatmentinvolves a heating of the glass product for a period of 5 minutes to 1hour at a first temperature of 450 to 550° C. In some cases it isconvenient to combine step b) and step c) in order to relax the glassarticle and nucleate the lithium metasilicate crystals in one singleheat treatment therefore a process as described above, wherein step (c)is replaced by modifying step (b) such that during the cooling process atemperature of about 450 to 550° C. is held for a period of about 5minutes to 50 minutes to produce the glass product which contains nucleisuitable for formation of the lithium metasilicate crystals during step(b) forms a preferred embodiment of the invention.

A process as described above, wherein in step (c) the first heattreatment comprises heating the starting glass blank to a temperature ofabout 450 to 550° C. for a period of about 5 minutes to 1 hour formsanother preferred embodiment of the invention.

Subsequently, the glass product comprising the desired nuclei is cooledto room temperature.

In the subsequent step (d), the glass product having the desired nucleiof Li₂SiO₃ is subjected to a second heat treatment at a secondtemperature which is higher than the first temperature. This second heattreatment results in the desired formation of lithium metasilicatecrystals as predominant and preferably as only crystalline phase andtherefore gives the lithium metasilicate glass ceramic according to theinvention. Preferably, this second heat treatment of step (d) comprisesheating the glass product which contains nuclei suitable for formationof lithium silicate crystals to a second temperature of about 600 to700° C. for a period of about 10 to 30 minutes.

The principle temperature profile of such a process is exemplified inFIG. 1. Already starting from the melt (1), i.e. at the end of step a)the temperature decreases for relaxation of the product in a temperaturerange of 500 to 450° C. (2). The temperature can then be brought to roomtemperature (solid line), step b), and afterwards be brought to atemperature of about 450 to 550° C. or can be kept in the temperaturerange of 450 to 500° C. (dotted line). In the region that is labelledwith (3), step c), nucleation occurs at a temperature of 450 to 550° C.and is influenced by P₂ 0 ₅. Then, the glass material can be heateddirectly to a temperature in the range of 600 to 700° C. and kept atsaid temperature (4) during which time lithium metasilicate forms, stepd). Subsequently the material can be cooled down (solid line) to e.g.about room temperature for grinding, milling or CAD-CAM processing andcan afterwards be brought to a temperature of about 700 to 950° C. orcan directly be brought to 700 to 950° C. (dotted line) at whichtemperature (5) the second crystallization occurs forming the lithiumdisilicate and where additional heat treatment or hot pressing can beundertaken.

Depending on the specific composition of a selected starting glass, itis possible for the skilled person by means of differential scanningcalorimetry (DSC) and x-ray diffraction analyses to determine suitableconditions in steps (c) and (d) to result in materials having thedesired morphology and size of the crystals of lithium metasilicate. Tofurther illustrate this process FIGS. 2 to 5 together with Tables I andII in the example section indicate how the relevant data were obtainedfor example 13 using said measurements and are therefore obtainable ingeneral. Moreover, these analyses allow also the identification ofconditions avoiding or limiting the formation of undesirable othercrystalline phases, such as of the high-strength lithium disilicate, orof cristobalite and lithium phosphate.

Subsequent to step (d), it is preferred to shape the obtained glassceramic. This is preferably effected by step (e), wherein the lithiummetasilicate glass ceramic is machined to a glass ceramic product of thedesired shape, in particular the shape of a dental restoration. Themachining is preferably carried out by trimming or milling. It isfurther preferred that the machining is controlled by a computer, inparticular by using CAD/CAM-based milling devices. This allows aso-called chair-side treatment of the patient by the dentist.

It is a particular advantage of the glass ceramic according to theinvention that it can be shaped by machining without the undue wear ofthe tools observed with the tough and high-strength prior art materials.This is in particular shown by the easy possibility to polish and trimthe glass ceramics according to the invention. Such polishing andtrimming processes therefore require less energy and less time toprepare an acceptable product having the form of even very complicateddental restorations. Lithium disilicate dental restorations can beproduced in many different ways. Commonly used by dental technicians arethe CAD/CAM and the hot pressing technique. Dentists can use a CAD/CAMmethod (Cerec 2®, Cerec 3®) to produce chair-side an all-ceramic lithiumdisilicate restoration. The final result is always a dental restorationwith lithium disilicate as the main crystalline phase. For this purpose,the blank can be a lithium metasilicate glass ceramic according to theinvention. The glass ceramic according to the invention can therefore beprocessed in both ways, by CAD/CAM or by hot-pressing, which is veryadvantageous for the user.

It is also possible to use for these purposes a corresponding lithiumsilicate glass which comprises nuclei suitable for formation of lithiummetasilicate crystals. This glass is a precursor of the lithiummetasilicate glass ceramic of the invention. The invention is alsodirected to such a glass. It is obtainable by the above process in step(c).

For manufacturing a dental restoration by the hot pressing technique alithium silicate glass ingot having nuclei for lithium metasilicate issubjected to a heat treatment of about 700 to 1200° C. to convert itinto a viscous state. The heat treatment will be conducted in a specialfurnace (EP 500®, EP 600®, Ivoclar Vivadent AG) . The ingot is embeddedin a special investment material. During the heat treatment the ingotwill be crystallized. The main crystal phase is then lithium disilicate.The viscous glass ceramic flows under a pressure of 1 to 4 MPa into thecavity of the investment material to obtain the desired shape of thedental restoration. After cooling the investment mould to roomtemperature the lithium disilicate restoration can be divested by sandblasting. The framework can be further coated with a glass or a glassceramic by sintering or hot pressing technique to get the finalizeddental restoration with natural aesthetics.

An ingot which comprises the lithium silicate glass ceramic according tothe invention is subjected to a heat treatment of about 700 to 1200° C.to convert it into a viscous state. The heat treatment will be conductedin a special furnace (EP 500®, EP 600®, Ivoclar Vivadent AG). The glassceramic ingot is embedded in a special investment material. During theheat treatment the glass ceramic will be further crystallized. The maincrystal phase is then lithium disilicate. The viscous glass ceramicflows under a pressure of 1 to 4 MPa into the cavity of the investmentmaterial to obtain the desired shape of the dental restoration. Aftercooling the investment mould to room temperature the lithium disilicaterestoration can be divested by sand blasting. The framework can befurther coated with a glass or a glass ceramic by sintering or hotpressing technique to get the finalized dental restoration with naturalaesthetics.

For manufacturing a dental restoration by the CAD/CAM technique thelithium silicate or the lithium metasilicate blocks with lithiumdisilicate as possible minor crystalline phase having a strength ofabout 80 to 150 MPa can be easily machined in a CAM unit like Cerec 2®or Cerec 3® (Sirona, Germany) . Larger milling machines such as DCSprecimill (DCS, Switzerland) are also suitable. The block is thereforepositioned in the grinding chamber by a fixed or integrated holder. TheCAD construction of the dental restoration is done by a scanning processor an optical camera in combination with a software tool. The millingprocess needs for one unit 10 to 15 minutes. Copy milling units such asCelay® (Celay, Switzerland) are also suitable for machining the blocks.First, a 1:1 copy of the desired restoration is fabricated in hard wax.The wax model is then mechanically scanned and 1:1 mechanicallytransmitted to the grinding tool. The grinding process is therefore notcontrolled by a computer. The milled dental restoration has to besubjected to a heat treatment to get the desired lithium disilicateglass ceramic with high strength and tooth like color. The heattreatment is conducted in the range of 700 to 900° C. for a period ofabout 5 to 30 minutes. The framework can be further coated with a glassor a glass ceramic by sintering or hot pressing technique to get thefinalized dental restoration with natural aesthetics.

Blocks with lithium disilicate as main crystalline phase can only begrinded in a large milling machine such as DCS precimill® (DCS,Switzerland) due to the high strength and toughness of the glassceramic. The block is therefore positioned in the grinding chamber by afixed metal holder. The CAD construction of the dental restoration isdone by a scanning process in combination with a software tool. Anadditional heat treatment in the range of 700 to 900° C. could beconducted in order to close surface flaws which were induced by thegrinding process. The framework can be further coated with a glass or aglass ceramic by sintering or hot pressing technique to get thefinalized dental restoration with natural aesthetics.

It has further been shown that the easily machinable lithiummetasilicate glass ceramic according to the invention can be convertedinto a lithium disilicate glass ceramic product by a further heattreatment. The obtained lithium disilicate glass ceramic has not onlyexcellent mechanical properties, such as high strength, but alsodisplays other properties required for a material for dentalrestorations.

Thus, the invention also relates to a process for preparing a lithiumdisilicate glass ceramic product, which comprises

(f) subjecting the lithium metasilicate glass ceramic according to theinvention to a third heat treatment to convert lithium metasilicatecrystals to lithium disilicate crystals.

In this step (f), a conversion of the metastable lithium metasilicatecrystals to lithium disilicate crystals is effected. Preferably, thisthird heat treatment involves a complete conversion into lithiumdisilicate crystals and it is preferably carried out by heating at 700to 950° C. for 5 to 30 minutes. The suitable conditions for a givenglass ceramic can be ascertained by conducting XRD analyses at differenttemperatures.

It was also found out that the conversion to a lithium disilicate glassceramic is associated with only a very small linear shrinkage of onlyabout 0.2 to 0.3%, which is almost negligible in comparison to a linearshrinkage of up to 30% when sintering ceramics.

A process as described above, wherein the lithium silicate blank has abiaxial strength of at least 90 MPa and a fracture toughness of at least0.8 MPam^(0.5) is preferred.

A process as described above, wherein the starting glass blank of step(b), the glass product containing nuclei suitable for forming lithiummetasilicate of step (c), or the lithium silicate blank with lithiummetasilicate as the main crystalline phase of step (d) is shaped to adesired geometry by machining or by hot pressing to form a shapedlithium silicate product is also preferred.

Such a process, wherein the shaped lithium silicate blank is a dentalrestoration is more preferred and a process wherein the dentalrestoration is an inlay, an onlay, a bridge, an abutment, a facing, aveneer, a facet, a crown, a partial crown, a framework or a coping iseven more preferred.

A process as described above, wherein the machining is performed bygrinding or milling forms a preferred embodiment of the invention,whereby a process wherein the machining is controlled by a computer iseven more preferred.

A process as described above but further comprising subjecting theshaped lithium silicate product to a third heat treatment at a thirdtemperature of about 700 to 950° C. for a period of about 5 to 30minutes is another aspect of the present invention and said process isparticularly preferred when the lithium silicate product subjected tothe third heat treatment comprises lithium metasilicate as the maincrystalline phase, and wherein the third heat treatment converts thelithium metasilicate crystals to lithium disilicate crystals as the maincrystalline phase of the dental restoration.

A process as described above wherein the lithium silicate productsubjected to the third heat treatment comprises the glass productcontaining nuclei suitable for forming lithium metasilicate crystals,and wherein lithium disilicate crystals are crystallized directly fromthe nuclei suitable for forming lithium metasilicate crystals is alsopreferred.

Another preferred embodiment of the present invention is a process asdescribed above, wherein the shrinkage that occurs during the third heattreatment is less than 0.5%, preferably less than 0.3%, by volume.

A process as described above which comprises shaping of a lithiumsilicate material to the desired geometry by hot pressing to produce thedental restoration is also an object of the invention, with a processfor manufacturing a dental restoration as described above beingpreferred wherein the hot pressing comprises subjecting the lithiumsilicate material to a heat treatment at a temperature of about 500 to1200° C. to convert the lithium silicate material into a viscous stateand pressing the viscous lithium silicate material under a pressure ofabout 1 to 4 MPa into a mould or dye to obtain the dental restorationwith a desired geometry.

A process as described above, wherein the lithium silicate materialsubjected to the heat treatment and pressing comprises lithiummetasilicate crystals which are converted into lithium disilicatecrystals during the heat treatment and pressing is more preferred.

A further preferred embodiment of the present invention is formed by aprocess as described above which comprises an increasing of strength andfracture toughness of the lithium silicate material.

A process for the manufacture of a dental restoration as described aboveis preferred, wherein the dental restoration has a biaxial strength ofat least 250 MPa and a fracture toughness of at least 1.5 MPam^(0.5).

A process for the manufacture of a dental restoration as described abovefurther comprising finishing the dental restoration to obtain a naturalappearance is preferred.

Same is true for a process as described above, wherein the finishingstep comprises applying a coating to the dental restoration by layeringwith powdered materials or by hot pressing a coating material onto theunfinished dental restoration.

A process as described above wherein the third heat treatment occursduring a firing of the layering materials or the hot pressing of thecoating material onto unfinished the dental restoration is even morepreferred.

Thus, a product is finally obtained which has all the beneficialmechanical, optical and stability properties making lithium disilicateceramics attractive for use as dental restorative materials. However,these properties are achieved without the disadvantages of theconventional materials when shaped by using a CAD/CAM based process, inparticular the undue wear of the milling and trimming tools.

Consequently, the invention also relates to a lithium disilicate glassceramic product which is obtainable by the above process for itspreparation and has lithium disilicate as main crystalline phase.Preferably, the lithium disilicate glass ceramic product according tothe invention is in the form of a dental restoration.

It is further preferred that in the lithium disilicate glass ceramic thelithium disilicate crystals form 60 to 80% by volume of the glassceramic.

The conversion of the lithium metasilicate glass ceramic according tothe invention to a lithium disilicate glass ceramic product isassociated with a surprisingly high increase in strength by a factor ofup to about 4. Typically, the lithium metasilicate glass ceramic of theinvention has a strength of about 100 MPa, and the conversion leads to alithium disilicate glass ceramic having a strength of more than 400 MPa(measured as biaxial strength).

The invention is also directed to a lithium silicate blank as describedabove, wherein the holder is jointed and connected with the holder.

A lithium silicate blank as described above, wherein the holder is froma different material from the blank forms one embodiment of theinvention.

A lithium silicate material blank as described above, wherein the holderis made from an alloy, from a metal, from a glass ceramic or from aceramic forms a preferred embodiment of the invention.

A lithium silicate blank as described above, wherein the holder is madefrom the same material as the blank and is integral with the blank isanother embodiment of the invention.

A lithium silicate blank as described above, wherein the blank islabelled with information is another preferred embodiment.

Same is true for a lithium silicate blank as described above, whereinthe information on the blank comprises the material, the size and thetype of the shape, which is to be machined from the blank.

Another aspect of the present invention is directed to a method formanufacturing a lithium silicate restoration comprising preparing alithium silicate blank as described above, and thereafter coating adental restoration with the lithium silicate blank.

A method for manufacturing a dental restoration as described abovewherein a dental framework is coated by hot pressing the lithiumsilicate blank onto the dental framework is preferred.

A method for manufacturing a dental restoration as described above,wherein the dental framework is a crown, a partial crown, a bridge, acoping, a veneer, a facing or an abutment is more preferred and such amethod, wherein the dental framework is made from a metal, an alloy, aceramic or a glass ceramic is even more preferred.

A method for manufacturing a dental restoration as described above,wherein the ceramic comprises zirconium oxide, aluminium oxide, azirconium mix oxide, an aluminium mix oxide, or a combination thereofforms a particularly preferred embodiment of the invention.

A method for manufacturing a dental restoration as described abovewherein the lithium silicate blank which is coated onto the frameworkcomprises lithium metasilicate crystals which are converted to lithiumdisilicate crystals, or the lithium silicate blank comprises nucleisuitable for forming lithium metasilicate crystals which crystallize aslithium disilicate crystals during the hot pressing of the lithiumsilicate blank onto the dental framework is another preferred object ofthe invention.

The invention is explained in more detail below on the basis ofExamples.

EXAMPLES Examples 1 to 18 (Invention), 19 to 20 (Comparison) and 21 to23 (Invention)

A total of 18 different lithium metasilicate glass ceramic productsaccording to the invention as well as two ceramics for comparison withthe chemical compositions given in Table III were prepared by carryingout stages (a) to (d) of the process described above and finallyconverted to lithium disilicate glass ceramic products by step (e) ofthe process described above:

For this purpose samples of the corresponding starting glasses weremelted in a platinum-rhodium crucible at a temperature of 1500° C. andfor a period of 3 hours (a).

The glass melts obtained were then poured into steel moulds which werepreheated to 300° C. After 1 minute the glass blanks were transferredinto a furnace which was preheated to a temperature between 450 and 550°C. The exact values, KB T [° C.] and KB t [min], are given for eachsample in Table III. After this relaxation and nucleation process (b andc) the blocks were allowed to cool to room temperature. The nucleatedsamples were homogeneous and transparent.

The glass blanks, which contained nuclei for the crystallization, werethen subjected to step (d), i.e. the second heat treatment, tocrystallize lithium metasilicate, which means that the glass blanks wereexposed to a temperature of about 650° C. for a period of about 20minutes, except example 3, which was crystallized at 600° C.

The course of the crystallization was investigated by DSC-measurementand the resulting crystal phases were analyzed by XRD to identify theideal conditions for this heat treatment. “Ideal conditions” in thesense of the present invention are present in case the twocrystallization peaks of the meta- and the disilicate phase respectivelyare differing to such an extend that in the production process a neatdifferentiation can be implemented, i.e. when heating a sample to thefirst crystallization temperature it has to be secured that whenreaching the desired temperature within the sample the temperature atthe outer regions of the sample does not reach the secondcrystallization temperature, i.e. the bigger the temperature differenceof the first and the second crystallization temperature is the biggerthe sample mass can be.

To further illustrate the process FIG. 2 shows a DSC-plot of one of theexamples, example 13, a quenched and powdered glass sample, which washeated with a heating rate of 10 K/min. The crystallisation of lithiummetasilicate (1), the crystallisation of lithium disilicate (2) as wellas the glass transition temperature (3) and the temperature range (4)for the first crystallisation are clearly visible from said DSC-plot.

Also an example for the analysis of phase development by hightemperature XRD from the same example 13 is given. FIG. 3 thereforeshows the measurement of a bulk glass sample at a constant heating rateof 2 K/min. It can be recognized from said measurement that in this casethe crystallisation of the lithium metasilicate (1) occurs at atemperature of 510° C. and that in this case the resolution of thelithium metasilicate and the crystallization of the lithium disilicate(2) occur at a temperature of 730° C.

FIG. 4 represents a phase analysis by XRD of example 13 after nucleationat 500° C. for 7 min and first crystallisation at 650° C. and 20 min.

The corresponding data are summarized in Table I:

TABLE I 1 2 d-spacing in 0.1 d-spacing in 0.1 nm 3 nm of scan of patternIndex 4.628 4.690 LS 020 3.296 3.301 LS 111 2.708 LS 130 2.685 2.700 LS200 2.355 2.342 LS 131 2.333 2.331 LS 002

FIG. 5 shows an SEM-micrograph, backscattered electrons, of the sameexample having the same thermal history, with the surface being etchedwith 1% HF for 8 s. Clearly visible are holes that show former lithiummetasilicate crystals.

The resulting blocks were now ready for step (e), which means shapingthe lithium metasilicate glass ceramic to the desired shape, either bysaw cutting, or by milling it in a CAD-CAM milling machine (i.e. CEREC3®) . The obtained lithium metasilicate glass ceramic blanks wereanalyzed for their machinability and their edge strength. 10 discs werecut from a rod with 12 mm diameter for biaxial strength measurements.The results of these analyses are given in Table IV. Ten more discs wereprepared and subjected to a third heat treatment (f).

In case the blanks contain colouring and fluorescent oxides the blocksin the state of the metasilicate appear to have a reddish or bluishcolour. This effect vanishes when the disilicate phase forms and theblanks turn to the colour that is desired.

Finally, the lithium metasilicate glass ceramic blanks were subjected toa second crystallization, step (f), at 850° C. for 10 min, exceptexample 3 which was crystallized at 830° C., i.e. the third heattreatment which is in general performed at temperatures of 700 to 950°C., preferably 820 to 880° C. and for a period of 5 to 30 minutes,preferably 5 to 20 minutes, to convert the lithium metasilicate intolithium disilicate.

The obtained products were analyzed for their crystal phases. To furtherillustrate the procedure the phase analysis for example 13 afternucleation at 500° C. for 7 min, first crystallization at 650° C. for 20min and second crystallization at 850° C. for 10 is shown in FIG. 6. Thecorresponding data are summarized in Table II.

TABLE II 1 2 d-spacing in 0.1 nm d-spacing in 0.1 nm 3 of scan ofpattern Index 5.369 5.420 LS2 110 3.986 3.978 LP 120 3.855 3.834 LP 1013.714 3.737 LS2 130 3.629 3.655 LS2 040 3.562 3.581 LS2 111 2.929 2.930LS2 131 2.901 2.908 LS2 200 2.379 2.388 LS2 002 2.346 2.35 LS2 221 2.2832.29 LS2 151 2.050 2.054 LS2 241

FIG. 7 shows an SEM-micrograph, backscattered electrons, of the sameexample having the same thermal history, with the surface being etchedwith 3% HF for 30 s leading to the glassy phase being etched out andleaving the lithium disilicate crystals.

In addition to the analysis in respect to crystal phases the sampleswere also analyzed in respect to their biaxial strength and chemicaldurability. Furthermore, their translucence was assessed. The resultsare also given in Table IV.

In table IV, the detected crystalline phases are designated as follows:

-   LS—lithium metasilicate-   LS2—lithium disilicate-   LP—lithium phosphate,    with the main phase being marked in bold type.

To gain information about the machinability tests were performed on aCerec® 3, with new tools being used for each test. A ‘Lego®-Minicube’served as a model which had to be milled from all compositions that weresubjected to this test and from a leucite-enforced glass ceramic of thename ProCAD® from Ivoclar Vivadent AG. The operating sequence was asfollows: First a blank of ProCAD® was milled, then a blank of theceramic to be tested was milled and after that again a ProCAD® blank wasmilled. The machinability was rendered “very good” in case the time thatwas required to mill the blank of the ceramic to be tested was below 95%of the time that was required to mill the ProCAD® blank. Times in therange of 95 to 105% of said time led to the mark “good” for themachinability, times in the range of 105 to 115% to “acceptable” andtimes above 115% to “poor”. The medium time required for the millingprocess was 14.0 minutes.

To compare the machinability of the test samples with another glassceramic a blank made according to the composition disclosed in DE 197 50794 was prepared and subjected to the test described above. After 15minutes the test was abandoned since only about 10% of the volume to bemilled was already milled and the tools used for milling were alreadyworn out, something that did not happen with any of the test samples.

The edge strength was determined as follows:

With a milling unit (CEREC 3®) blanks were milled to result inLego-minicubes. With a 1.6 mm cylindrical diamond cutter blind holeswere milled. The quality of said blind holes was determined by comparingthe area of the broken out edges with those of a reference sample(ProCAD®). The relation of the area of the broken out edges to the areaof the blind bore is an allocation for the edge strength.

An edge strength is considered to be “very good” in case the relation ofsaid areas is smaller than that of the reference, it is considered to be“good” in case the relations are about the same and it is considered tobe “acceptable” in case the area is bigger than 110% of the referencesample.

The chemical durability was determined according to ISO 6872, i.e. asloss of mass after 16 h in 4% acetic acid at 80° C. “Good” means thatthe solubility according to said method is below 100 μg/cm².

The strength was measured as biaxial strength according to ISO 6872 oras 3 point bending strength according to EN 843-1:

Bars of 12 mm diameter were casted and crystallized once. From thesebars 20 discs with a thickness of 1,2 mm each were sawn. 10 of thesediscs were then smoothed and the surfaces of the discs were polishedusing SiC-paper of grain size 1000. Biaxial strength was measured as isdisclosed in ISO 6872. The other 10 discs were crystallized a secondtime at 800 to 900° C. to give the lithium disilicate phase. Thesesolidified samples were smoothed on both sides and the surfaces werepolished using SiC-paper of grain size 1000. Biaxial strength was thenmeasured according to ISO 6872.

By comparison bending strength was measured on bars with dimensions of25 * 3.5 * 3.0 mm were sawn out of a block of the lithium metasilicateglass ceramic. These bars were smoothed to result in bars havingdimensions of 25 * 2.5 * 2.0 mm which were then polished using SiC-paperof grain size 1000. The edges were also bevelled with SiC-paper of grainsize 1000. The span was 20 mm. The results are comparable to biaxialstrength results.

In addition to this, fracture toughness was determined by applying aVickers indentation onto a polished surface and measuring the size ofthe flaws originating from the edges (Indentation Force Method . . .IF). This method is useful as comparative method but does not result inabsolute values. For comparison measurements were performed on notchedbending samples (SENB, SEVNB). For the lithium disilicate glass ceramicsfracture toughness values>2 MPam^(0.5) were obtained.

In Table II the values for the biaxial strength and the fracturetoughness of the samples having the disilicate phase, i.e. those samplesthat were crystallized twice, are given. In addition to that quotientsare given which give the ratio of the biaxial strength of the disilicatesystem to the biaxial strength of the metasilicate system (biaxialsolidification factor) or the ratio of the fracture toughness of thedisilicate system to the fracture toughness of the metasilicate system(solidification factor KlC).

Translucence was determined after the second crystallization: a testpiece 16 mm diameter and having a thickness of 2 mm was prepared andpolished on both sides. The contrast value CR was determined accordingto BS 5612 (British Standard) using a spectral calorimeter (MinoltaCM-3700d). The determination of the contrast value consisted of twosingle measurements. The test piece to be analyzed is therefor placed infront of a black ceramic body having a reflexion of 4% at most andaccordingly in front of a white ceramic body having a reflexion of 86%at minimum which are then colourmetrically determined. Using highlytransparent test pieces reflexion/absorption is mainly caused by theceramic background whereas reflexion is caused by the test piece in casean opaque material is used. The ratio of reflected light on black groundto reflected light on white ground is the quantum for the contrastvalue, with total translucence leading to a contrast value of 0 andtotal opaquescence leading to a contrast value of 1. The samples wererated as follows:

extraordinary: CR < 0.4 very good: 0.4 < CR < 0.5 good: 0.5 < CR < 0.6acceptable: 0.6 < CR < 0.8 opaque: 0.8 < CR.

TABLE III Expl No. 1 2 3 4 5 6 7 8 9 10 KB T 500 490 520 500 500 500 500500 500 500 [° C.] KB t 10 30 5 30 10 10 10 10 10 10 [min] wt % SiO₂69.3 73.0 64.0 68.1 70.1 69.0 68.6 69.9 68.6 68.8 K₂O 4.3 4.4 4.2 4.24.5 4.3 4.3 4.4 2.0 5.0 Na₂O 2.0 SrO 2.0 BaO 2.0 2.0 CaO 2.0 Li₂O 15.317.0 13.0 15.0 15.5 15.2 15.1 15.4 15.1 15.1 Al₂O₃ 1.1 1.1 4.0 5.0 1.11.1 1.1 1.1 3.0 1.1 P₂O₅ 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 5.0 3.8 MgO 1.00.0 1.0 0.0 5.0 1.0 1.0 1.0 0.0 1.0 ZrO₂ 2.0 ZnO 5.2 0.7 6.0 3.9 0.0 3.64.1 2.4 4.3 3.2 TiO₂ V₂O₆ Fe₂O₃ MnO₂ CeO₂ 2.0 YaO₃ La₂O₃ Pr₂O₃ Ta₂O₆Tb₄O₇ Er₂O₃ 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0Expl No. 11 12 13 14 15 16 17 18 19 20 KB T 520 500 500 500 500 500 500500 500 500 [° C.] KB t 10 10 7 7 7 10 20 10 10 30 [min] wt % SiO₂ 70.065.7 67.4 68.4 65.0 70.0 70.0 67.8 68.3 67.7 K₂O 5.0 4.1 4.0 2.7 2.0 4.43.8 4.1 4.3 4.2 Na₂O 0.1 1.0 0.1 0.1 0.1 0.1 SrO 2.0 BaO 2.0 CaO 1.0Li₂O 15.0 14.5 14.8 15.0 14.0 16.0 16.0 15.0 15.1 14.9 Al₂O₃ 1.1 1.1 1.13.0 4.1 1.8 1.1 1.1 0.0 0.0 P₂O₅ 2.0 3.8 3.8 3.5 3.8 3.8 3.8 3.8 3.8 3.8MgO 0.9 1.0 0.5 0.1 0.0 0.3 0.1 0.1 1.0 1.0 ZrO₂ 1.0 0.1 0.1 0.1 0.1 0.10.1 ZnO 6.0 2.8 4.7 5.2 4.0 2.0 4.5 4.8 5.1 5.0 TiO₂ 1.6 V₂O₆ 0.2 Fe₂O₃0.2 MnO₂ 0.2 0.5 CeO₂ 0.5 2.0 1.0 0.4 1.0 0.4 0.5 YaO₃ 2.4 La₂O₃ 0.5 0.31.0 0.1 0.1 0.3 3.4 Pr₂O₃ 1.0 Ta₂O₆ 1.5 Tb₄O₇ 1.5 0.5 0.5 0.5 Er₂O₃ 1.00.3 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

TABLE IV Ex. No. 1 2 3 4 5 6 7 8 9 10 phases present after LS LS, LS2 LSLS LS, LS2 LS LS LS, LS2 LS LS 1st crystallisation phases present afterLS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LPLS2, LP 2nd crystallisation biaxial strength after 359 424 250 314 324472 426 404 356 319 2nd crystallisation biaxial solidification 3.0 2.42.5 3.4 2.4 3.5 3.5 2.3 3.2 2.7 factor K1C [MPm^(0.5)] after 1.6 2.2 1.91.9 1.8 2.3 1.8 2.4 1.9 1.8 2nd crystallisation K1C solidification 1.81.7 2.6 2.5 1.6 2.4 2.0 1.9 1.9 1.8 factor grinding time in 93% 103% 95%89% 98% 93% 94% 105% 94% 94% comparison to ProCAD machinability verygood good very good very good good very good very good good very goodvery good edge strength good very good good good good good acceptablegood acceptable good translucency very good n.m. n.m. n.m. n.m.extraordi- n.m. n.m. acceptable n.m. chemical durability good good goodgood good good good good good good (ISO 6872) Ex. No. 11 12 13 14 15 1617 18 19 20 phases present after LS, LS2 LS LS LS LS LS LS LS LS2 LS21st crystallisation phases present after LS2, LP, LS2, LP LS2, LP LS2,LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP 2nd crystallisationbiaxial strength after 301 354 381 389 342 329 420 387 440 405 2ndcrystallisation biaxial solidification 1.7 2.9 3.0 3.1 2.6 2.9 3.2 3.42.2 2.1 factor K1C [MPm^(0.5)] after 2.1 2.0 1.9 2.0 1.8 17 2.1 1.9 1.81.9 2nd crystallisation K1C solidification 1.6 1.9 2.1 1.8 2.0 1.6 2.21.9 1.0 1.5 factor grinding time in 115% 98% 90% 91% 94% 100% 95% 94%119% 129% comparison to ProCAD machinability acceptabl very good verygood very good very good good good very good poor poor Sedge strengthgood acceptabl very good very good very good very good very good goodgood good translucency n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m.chemical durability good good good good good good good good good good(ISO 6872)

The data in Table II show that the lithium metasilicate glass ceramicscombine a very good machinability and high edge strength with the easypossibility to convert them by a simple heat treatment into lithiumdisilicate glass ceramics which have a very high bending strength aswell as an excellent chemical durability and good translucence, all ofwhich being properties which make them very attractive as materialsuseful for the manufacture of dental restorations.

In the following some examples are described in more detail:

Example 1

The glass was molten at a temperature of 1500° C. for 3 hours and wasthen poured into steel moulds which were preheated to 300° C. After oneminute the glass bars were transferred into a cooling furnace and weretempered at 500° C. for 10 minutes and then cooled to room temperature.

The glass was homogeneous and transparent.

Following the glass bar was subjected to a first crystallization at 650°C. for a period of 20 minutes.

From the such ceramized bar, discs were sawn out of a round bar, andbiaxial strength was measured. The phase content was analyzed via XRD(X-ray diffraction). Lithium metasilicate was the only phase that wasdetected. Biaxial strength was 119+/−25 MPa.

Also the milling time of test bodys was measured. The milling time ofthe test body was one minute below that of ProCAD®, which was used asreference.

The edge strength was good.

Additional 10 discs were subjected to a second crystallization at 850°C. for a period of 10 minutes and biaxial strength and fracturetoughness were measured.

Biaxial strength was 359+/−117 MPa which correlates to a solidificationfactor of 3.0.

Fracture toughness (IF) was 1.6 MPam^(0.5).

Translucence was very good.

The chemical stability according to ISO 6872 (4% acetic acid, 80° C., 16h) was 37 μg/cm².

Example 6

Glass bars were produced according to example 1. The glass again washomogeneous and transparent.

The first crystallization was performed at 650° C. for a period of 20minutes.

Lithium metasilicate was determined to be the main phase with traces oflithium disilicate also being present. Biaxial strength was 135+/−24MPa.

Again the milling time of a test body was measured. The milling time ofthe test body was one minute below that of ProCAD®, which again was usedas reference.

The edge strength was very good.

After a second crystallization which was performed according to example1 the biaxial strength was 472+/−85 MPa which correlates to asolidification factor of 3.5.

Fracture toughness (IF) was 2.3 MPam^(0.5).

Translucence was extraordinary.

Example 9

Glass bars were produced according to example 1. The glass again washomogeneous and transparent.

The first crystallization was performed at 650° C. for a period of 20minutes.

Lithium metasilicate was determined to be the only phase. Biaxialstrength was 112+/−13 MPa.

Again the milling time of a test body was measured. The milling time ofthe test body was one minute below that of ProCAD®, which again was usedas reference.

The edge strength was good.

After a second crystallization which was performed according to example1 the biaxial strength was 356+/−96 MPa which correlates to asolidification factor of 3.16.

Fracture toughness (IF) was 1.9 MPam^(0.5).

Translucence was acceptable.

Example 20 (Comparison)

Glass bars were produced according to example 1. The glass again washomogeneous and transparent.

The first crystallization was performed at 650° C. for a period of 20minutes.

Lithium disilicate was determined as the main phase and lithiummetasilicate was only present in traces. Biaxial strength was 194+/−35MPa.

Again the milling time of a test body was measured. The milling time ofthe test body was four minutes longer that of ProCAD®, which again wasused as reference.

The edge strength was poor.

After a second crystallization which was performed according to example1 the biaxial strength was 405+/−80 MPa which correlates to asolidification factor of 2.09.

Fracture toughness (IF) was 1.88 MPam^(0.5).

Translucence was very good.

This example makes it even more obvious that in the light of the glassceramic materials according to the invention the adverse properties inrespect to machinability of the prior art material disqualify same to beused in applications as are mentioned above.

The following examples 21 to 23 show the usefulness of the lithiumsilicate glass according to the invention which comprises nucleisuitable for the formation of lithium metasilicate and subsequentlylithium disilicate glass ceramics.

Example 21

A glass melt having the composition according to example 14 was meltedin a platinum crucible at a temperature of 1500° C. for 3 hours. Theglass was not poured in a steel mould, but quenched in water. Thus, aglass granulate formed which was dryed and subsequently heated to 500°C. for 30 minutes to produce nuclei suitable for the formation oflithium metasilicate crystals. The obtained glass was milled to aparticle size of less than 45 μm.

The obtained powder was mixed with a modelling liquid consisting of morethan 95% water and additives for improving moldability and layered on acrown cap of densely sintered zirconium oxide, e.g. DCS-zircon.

The crown cap was fired in a dental furnace at a temperature of 900° C.with 2 minutes holding time. By this procedure the applied glass powdercontaining nuclei for the crystallization was simultaneouslycrystallized and densely sintered so that a dentine core of lithiumdisilicate glass ceramic resulted. On this core a suitable incisal masshaving a suitable expansion coefficient was applied.

The final anterior tooth restauration showed good resistance againstrapid temperature changes up to 160° C. This proves a good bond betweenthe dentine layer of lithium disilicate glass ceramic and the frameworkof high-strength zirconium oxide.

Example 22

Bars of a glass having a composition according to example 14 wereprepared in the same manner as in example 1. The glass bars werehomogenous, transparent and light yellow coloured. A crown cap ofdensely-sintered zirconium oxide was circularly reduced. Subsequently adentine core was layered with dental wax and the crown margin wasmodeled on the stump. The restauration was provided with a cast-onchannel. The crown cap was applied on a muffle basis and embedded ininvestment material, (Empress Speed, Ivoclar). After the requiredbinding time the muffle was preheated to 850° C. resulting in theremoval of the wax. After 90 minutes a blank of the above lithiumsilicate glass having nuclei for forming lithium metasilicate was put inthe muffle and pressed on the cap of zirconium oxide in accordance withthe known Empress-hot pressing process at 900° C. This resulted incrystallization of the glass blank to a lithium disilicate glassceramic.

After divesting, the final product was a zirconium oxide cap having adentine layer of lithium disilicate glass ceramic. This dentalrestoration showed an excellent fit of the circular edge on the model.Furthermore, the so-prepared dentine layer was free from porosities.

Example 23

A metal cap of an alloy having an expansion coefficient of 12.8* 10⁻⁶¹/K and a solidification temperature of 1100° C. was prepared in a castprocess, sand-blasted and by an oxidation-firing step prepared for thefurther processing.

In an analogous manner as in example 22 a dentine core was applied onthe cap using modeling wax. The metal cap was embedded, and the wax wasremoved by firing in a furnace. As in example 22 a blank of the lithiumsilicate glass having suitable nuclei was hot-pressed on the metal capat 900° C.

The so-prepared dental restoration showed a good bond between metalframework and the lithium disilicate glass ceramic and also had a highresistance against drastic temperature changes of above 160° C.

1. A process for the preparation of a lithium silicate blank, whichcomprises (a) producing a melt of a starting glass containing theinitial components SiO₂, Li₂O, K₂ 0, Al₂ 0 ₃ and P₂ 0 ₅ as the maincomponents (b) pouring the melt of the starting glass into a mould toform a starting glass blank and cooling the glass blank to roomtemperature, (c) subjecting the starting glass blank to a first heattreatment at a first temperature to give a glass product which containsnuclei suitable for forming lithium metasilicate crystals, (d)subjecting the glass product of step (c) to a second heat treatment at asecond temperature which is higher than the first temperature to obtainthe lithium silicate blank with lithium metasilicate crystals as themain crystalline phase.
 2. A process according to claim 1, wherein thestarting glass of step (a) further comprises ZnO, Na₂O, Me^(II)O, ZrO₂,and coloring and fluorescent metal oxides, with Me^(II)O being one ormore members selected from the group consisting of CaO, BaO, SrO andMgO.
 3. A process according to claim 1, wherein the starting glass ofstep (a) comprises the following initial components, independently ofone another, in the following amounts Component wt. -% SiO₂ 65.0-70.0Li₂O 14.0-16.0 K₂O 2.0-5.0 Al₂O₃ 1.0-5.0 P₂O₅ 2.0-5.0 ZnO 2.0-6.0 Na₂O0.1-2.0 Me^(II)O 0.1-7.0 ZrO₂ 0.1-2.0 coloring and fluorescent 0.5-3.5metal oxides

with Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO and with the metal(s) of the one ormore coloring and fluorescent metal oxides being selected from the groupconsisting of Ta, Tb, Y, La, Er, Pr, Ce, Ti, V, Fe and Mn.
 4. A processaccording to claim 1, wherein step (c) is replaced by modifying step (b)such that during the cooling process a temperature of about 450 to 550°C. is held for a period of about 5 minutes to 50 minutes to produce theglass product which contains nuclei suitable for formation of thelithium metasilicate crystals.
 5. A process according to claim 1,wherein in step (c) the first heat treatment comprises heating thestarting glass blank to a temperature of about 450 to 550° C. for aperiod of about 5 minutes to 1 hour.
 6. A process according to claim 1,wherein the second heat treatment of step (d) comprises heating theglass product which contains nuclei suitable for formation of lithiumsilicate crystals to a second temperature of about 600 to 700° C. for aperiod of about 10 to 30 minutes.
 7. A process according to claim 1,wherein the lithium silicate blank has a biaxial strength of at least 90MPa and a fracture toughness of at least 0.8 MPam^(0.5).
 8. A processaccording to claim 1, further comprising (e) subsequently converting thelithium silicate blank product of step (d) to lithium disilicate.
 9. Aprocess according to claim 8, wherein the lithium disilicate product ofstep (e) is a dental product.
 10. A process according to claim 1,wherein the blank further comprises a holder to fix it in a machine. 11.A process according to claim 10, wherein the holder is jointed andconnected with the blank.
 12. A process according to claim 10, whereinthe holder is made from a different material than the blank.
 13. Aprocess according to claim 10, wherein the holder is made from an alloy,from a metal, from a glass ceramic or from a ceramic.
 14. A processaccording to claim 10, wherein the holder is made from the same materialas the blank and is integral with the blank.
 15. A process according toclaim 10, wherein the blank is labelled with information.
 16. A processaccording to claim 15, wherein the information on the blank comprisesthe material, the size and the type of the shape, which is to bemachined from the blank.
 17. A process according to claim 3, whereinMe^(II)O is present in an amount of from 0.1-5.0 wt.-%.